Infrared (IR) detectors are a type of thermal sensor utilized in a variety of fields (e.g., military, scientific, security/law-enforcement, medical, industrial and automotive) to detector IR radiation. Common applications using infrared detectors include rail safety, gas leak detection, flame detection, alcohol level testing for DUI's, anesthesiology testing, petroleum exploration, space operations, temperature sensing, water and steel analysis. The two main types of infrared detectors include thermal infrared detectors and optical (photonic) detectors. Thermal infrared detectors (e.g., microbolometers, discussed further below) utilize various approaches to detect IR radiation by way of measuring the thermal effects of the incident IR radiation using various temperature dependent phenomena. To date, optical methods are recognized as the most reliable detection techniques, with unparalleled sensitivities and robust spectral discrimination. However, today's commercial multispectral imaging systems typically utilize expensive and bulky spectrometers (e.g. Fabry-Pérot mirrors, FTIR, etc.), as well as very sensitive and expensive detectors (e.g. HgCdTe (MCT)), which must be cryogenically cooled.
Microbolometers are uncooled thermal sensor devices typically used as a detector in a thermal camera to measure the power of incident IR radiation with wavelengths between 7.5-14 μm via the heating of a material with a temperature-dependent electrical resistance. Each microbolometer consisting of an array of pixels, with an exemplary generalized conventional microbolometer pixel 50 being shown in FIG. 9. Microbolometer pixel 50 is fabricated on a semiconductor (e.g., silicon) substrate 51 along with associated readout circuitry using known techniques. Legs 52 are formed/patterned on the substrate surface, and then a reflector 53 is formed/patterned between legs 52. Next, a sacrificial layer (not shown) is deposited over the substrate surface to provide a process gap, and then a layer of IR absorbing material is deposited on the sacrificial layer and selectively etched such that opposite ends of the patterned IR absorbing material layer are attached to the upper ends of legs 52. To create the final bridge-like structure shown in FIG. 9, the sacrificial layer is then removed such that the absorbing material layer forms a membrane 54 that is suspended approximately 2 μm above the upper substrate surface. Because microbolometers do not undergo any cooling, absorbing material layer must be thermally isolated from the readout circuitry, which is achieved by the bridge-like structure. During operation, IR radiation is directed (e.g., using a camera lens) onto each pixel 50 and is absorbed by membranes 54, causing a change in resistance. Reflector 53 serves to redirect light passed through the IR absorbing material to ensure the greatest possible absorption. The resistance change across each membrane 54 is measured and processed into temperatures which can be used to create an image.
Although typical microbolometers are small and light, do not require cooling, and exhibit low power consumption, they typically exhibit lower sensitivity and resolution and higher noise (i.e., in comparison with cooled thermal and photon detector imagers), and they cannot be used for multispectral or high-speed infrared applications. Moreover, as mentioned above, the practical range of IR radiation wavelengths detectable by conventional microbolometers is currently 7.5 μm to 14 μm.
There are a variety of methods to detect methane leaks, ranging from manual inspection using trained dogs to advanced satellite-based hyperspectral imaging systems. However, the primary barrier to widespread deployment is cost. To date, optical detection techniques are widely recognized as the industry gold standard in view of their ability to decisively discriminate different gas species, as well as their high detection sensitivity. Moreover, optical methods are standoff techniques and use fewer sensors. The best optical methods are multispectral/hyperspectral detection methods—these methods use very expensive cryogenically cooled detectors and spectrometers. The Holy Grail is to achieve the same detection sensitivities and robust spectral discrimination with an uncooled thermal detector, and a smaller footprint spectrometer that is ideally integrated directly on the detector.
What is needed is an uncooled passive thermal sensor that can optically (remotely) detect IR radiation more accurately than conventional uncooled passive thermal sensors (e.g., microbolometers), has essentially the same production cost as conventional microbolometers, and can detect IR radiation having wavelengths below/above) the IR radiation wavelengths detectable by conventional microbolometers.
What is also needed is an uncooled multispectral IR imaging device having a detection sensitivity equal to or greater than existing multispectral/hyperspectral detection approaches that require expensive cryogenically cooled detectors and spectrometers.
What is also needed is a low-cost, highly reliable system and method for remotely detecting and measuring gas emissions (e.g., methane leaks).